Chartered Institution of Water and
Environmental Management (CIWEM)

Co-digestion of Sewage Sludge and Waste

Purpose

This Policy Position Statement outlines the main issues concerning
increasing the levels of co-digestion of waste together with sewage
sludge. It considers the technical, demographic, economic, legal
and policy barriers to wider co-digestion of waste. It
highlights areas where achievable opportunities exist and where
there is a requirement for greater facilitation, and considers the
overall contribution co-digestion can make to Anaerobic Digestion
(AD) targets. It is set within the wider context of the UK
Government's planned increased levels of AD and should be read in
conjunction with CIWEM's PPSs on Wastewater Biosolids Treatment and
Use[i].

CIWEM calls for:

A consistent and proportionate approach to policy for the
production and use of biogas irrespective of feedstock. This should
include appropriate financial incentives for the renewable energy
produced and rules for the land application of digestate.

A review and update of the regulations governing the outputs of
co-digestion, focused on the protection of soils, crops, animals,
people and the environment in general, from the perspective of the
receptors (not the origins of the feedstocks) and including clarity
on the end point for waste controls relating to co-digestate.

An update to the Quality Protocol for Anaerobic Digestate (PAS
110) to include provision for the use of biosolids (sewage sludge)
as feedstock in order to define a clear approved method for meeting
the requirements of Article 6 of the Waste Framework Directive on
End of Waste Criteria.

An appropriate and clear economic regulatory framework for
co-digestion, putting in place measures which permit water and
sewage companies to increase their biogas outputs via co-digestion
particularly where digester headroom capacity exists, reduce their
carbon impact and appropriately invest proceeds back into the
regulated business, potentially in cooperation with waste
companies.

The UK Government to provide and publish clarity on the Animal
By Product Regulations requirements for co-digestion.

Revision of the Sludge Use in Agriculture Regulations to
reflect the benefits of co-digestion and to put in place a clear
legislative framework for the use of all treated organic
residuals

The Chartered Institution of Water and Environmental
Management (CIWEM) is the leading professional body for the people
who plan, protect and care for the environment and its resources,
providing educational opportunities, independent information to the
public and advice to government. Members in 98 countries include
scientists, engineers, ecologists and students.

Context

Anaerobic digestion is a long-established technology for the
treatment of sewage sludge by the water industry. The technology is
of increasing interest to the waste management industry and to
farmers. DECC is promoting AD as a technology for renewable
energy and Defra for more sustainable waste management. AD
produces multiple benefits including a source of renewable energy
in the form of biogas (which can either be burnt to generate heat
and electricity in combined heat and power units or upgraded to
biomethane and injected into the gas distribution grid or used for
vehicle fuel). Digestate from the process can be used as a useful
fertiliser and soil conditioner for agriculture and other land use
applications. In addition, the recycling of digestate in this
way diverts biodegradable waste from landfill.

Defra[ii] highlights significant potential
for future AD capacity on the basis of over 100 million tonnes of
organic waste currently being produced in the UK annually,
comprising:

12-20 million tonnes of food waste (approximately half of which
is municipal waste collected by local authorities, the rest being
hotel or food manufacturing waste);

90 million tonnes of agricultural material such as manure and
slurry;

1.73 million dry tonnes of sewage sludge.

The UK Government has set a target of expanding AD capacity
significantly and hopes to see the UK as a world leader in
anaerobic digestion by 2020. Given the level of expertise on
AD within water companies, Defra regards this industry as being
central to the expansion of AD, thus the question of the ability to
feasibly co-digest sewage sludge with biodegradable waste is being
increasingly considered.

There are a number of targets for 2020 which help to drive the
high priority now attached to AD by the UK Government. Under the
Climate Change Act 2008[iii] greenhouse gas
emissions should be at least 34% below the 1990 baseline. In
addition, the EU Renewable Energy Directive[iv] requires 15% of UK's
energy to be generated from renewable sources. The 2009 Low Carbon
Transition Plan[v] and UK Renewable Energy
Strategy[vi] as well as the UK Biomass
Strategy[vii] and Waste Strategy for England[viii] (both 2007) also
encourage wider use of AD, including co-digestion. Production
of biogas by AD is also eligible for Renewables Obligation
Certificates (ROCs), although there are different rates for biogas
produced from anaerobically digested sewage sludge (which is
regarded as an established technology that receives 0.5 ROCs/ MWh)
and biodegradable solid waste (which receives 2.0 ROCs per MWh [and
3 or 4 ROCs depending on output in Northern Ireland]). The banding
of ROCs is due to be reviewed in 2011. The water industry
does not receive support for electricity generated under the
Feed-in Tariff scheme, though it will qualify for the Renewable
Heat Incentive when this is introduced in 2011.

In addition, there is an ongoing driver, in the form of the EU
Landfill Directive, to reduce the amount of biodegradable municipal
waste sent to landfill. By 2020 only 35% of the amount
produced in 1990 will be able to be disposed of this
way. Between 12 and 20 million wet tonnes of food waste
produced are produced in the UK per year, much of which would be
suitable for AD and at typical generation efficiencies, could
potentially be used to generate upwards of 6TWh of electricity
(roughly 1.7% of UK annual consumption).

Water and sewerage companies are presently responsible for by
far the largest inventory of AD infrastructure. Where any
spare (headroom) capacity exists, CIWEM considers that it would be
in the country's and the environment's interests to use this for
co-digestion. If the existing capacity is fully utilised it
could be 'turbo-charged' by retrofitting advanced digestion,
alternatively additional digesters could be built. All of
these options use the existing expertise, grid connections, vehicle
handling and onsite capacity to treat associated
effluent. A logical means of increasing such output may
be to co-digest sewage sludge with biodegradable material (e.g.
energy crop, manure or liquid waste) of an appropriate quality.

Co-digestion describes the process during which a number of
different waste types are mixed together and anaerobically
digested. This process can have clear benefits, firstly in
improving the ability to digest certain difficult to digest waste
types, and also, in the case of sewage sludge, adding to the
existing calorific value thus increasing the biogas output of the
digestion process. Importantly, in some locations it can
result in the critical mass of biogas substrate to be realised
within a haulage distance that makes AD financially
viable.

Many biodegradable wastes are co-digested (e.g. farmyard manure
plus municipal waste, food waste plus energy crops, etc.) but as
they all fall under a common regulatory framework, the main
obstacles relate to issues including demographics, investment
potential and feedstock suitability if AD is to be expanded
significantly as a waste treatment technique.

In the case of co-digesting sewage sludge (biosolids) with other
wastes (which is practiced in other countries including Denmark and
Germany) the UK has created additional complications by utilising
different regulatory regimes, which presently appear quite
difficult to reconcile, or which in order to satisfy, impose
economic burdens of such significance that they make such
co-digestion financially unattractive.

Discussion of Key Issues

Government targets

The UK Government sees AD as a viable source of renewable fuel
in the form of biogas, both for upgrading to biomethane for
injection into the grid or vehicle fuel and to generate
electricity. In addition, AD represents a means to divert
significant amounts of biodegradable waste away from landfill
provided an appropriate digestate recycling route is available.
Thus, it can contribute to meeting targets set under the Landfill
Directive (it must be emphasised that an appropriate recycling
route is essential for this diversion to be achieved - otherwise AD
outputs can result in a greater volume of waste produced than
existed initially because the waste has to be diluted before
digestion). Digestate also constitutes a useful source of
nutrients for recycling back to land. This context is likely
to see increased AD capacity in the UK in coming years, and the
Government has stated that it wishes the UK to become a world
leader in the technology.

A major attraction of co-digestion for the water industry is
its potential to make more use of existing AD assets and headroom
digester capacity, to optimise biogas output and help water
companies meet their own targets for renewable energy and energy
efficiency compliance. It may be argued that co-digestion
will not make any difference to the country's targets for
greenhouse gas reduction, landfill diversion or renewable energy,
because the feedstock employed in co-digestion would be exploited
by one of the many new digesters under construction or
planned. In addition, the tonnage of sewage sludge which
could be treated by AD (and thus co-digestion) is very limited
compared to the theoretical feedstock volumes available from other
sources. However this neglects some very significant issues:
sewage works' AD has on-site treatment for reject water, they have
grid connections and good road access, they have expertise to
operate AD and combined heat and power and they have 24-hour
surveillance monitoring. Defra envisages "The water
industry will be at the hub of a national anaerobic digestion
infrastructure. Where appropriate, water companies will generate
additional renewable energy by using their spare capacity to
process other feedstocks such as food waste."[ix]

Regulatory issues

The contribution to Government targets that can be made from
co-digestion of waste is limited under current circumstances.
The majority of existing AD capacity is operated by water and
sewerage companies and whilst a number are now examining the
potential to utilise their spare digester capacity by co-digesting
sewage sludge with other biodegradable wastes, there are a number
of regulatory barriers that prevent this potential from being
realised.

The digestate produced from sewage sludge is currently
controlled under the Sludge (Use in Agriculture) Regulations[x] which govern how it can
be applied to agricultural land. If appropriately produced
and managed (in accordance with the regulations), such digestate
represents a safe and inexpensive alternative to inorganic
fertiliser for farmers and a viable route for water companies as
well as a sustainable way of recycling nutrients to land.
These regulations only apply to agricultural use of digestate;
non-agricultural use is still governed by the Environmental
Permitting Regulations 2010[xi]. But there is
nothing in the Sludge Directive that would prevent it from being
relevant to all land application, provided there was separate
accounting for use in agriculture[xii].

Digestate from source-segregated biodegradable waste is
controlled under the Environmental Permitting Regulations and its
production may also be subject to requirements under the Animal
By-Products Regulations (ABPR)[xiii].
Spreading to agricultural land requires an environmental permit or
appropriate exemption for certain waste types. However, this
too could be appropriately managed under the tried and tested
sludge Regulations. Alternatively, an output produced
adhering to the requirements of approved quality protocols for
compost or anaerobic digestate (e.g. PAS 100 and PAS 110) may be
classified as a non-waste for re-use and not be subject to waste
regulation control.

If sewage sludge is co-digested with biodegradable waste, it
might no longer fall under the scope of the Sludge (Use in
Agriculture) Regulations and thus outside of the scope of the
Environmental Permitting Regulations. Consequently a
potentially expensive environmental permit may also be required of
the farmer in order for him to apply the resultant digestate to
land. Co-digestate is also presently not covered by any of
the approved quality protocols which also remove the output from
the regulatory regime for waste and in order for it to be covered
there will need to be clarity on how it could meet the requirements
of Article 6 of the EU Waste Framework Directive on End of Waste
Criteria. Also, additional pre-treatment of the feedstock may
be required to meet the requirements of the ABPR. Currently,
this has the effect of making co-digestion unattractively complex
and expensive, even in locations where there may be a locally
available, secure supply of feedstock of sufficiently high quality.
Under its Anaerobic Digestion Implementation Plan, Defra has
committed to identify the regulatory requirements for co-digestion
of sewage sludge with other feedstocks[xiv].

Investment issues

If water companies are to be encouraged to invest in AD of
waste, and co-digestion, there will need to be consideration as to
how companies can receive solid waste from producers, which are
outside their regulated customer base (with waste collection being
paid for via council taxes), utilise this to optimise biogas
production and reinvest the proceeds back into the regulated
business (which is funded through water bills). Waste
operators will also demand a level playing field so that they
are not at a competitive disadvantage to water companies, who may
be (or perceived to be) in a monopolistic position. Water companies
could theoretically make use of secure sources of funding (from
water bills) to develop digester capacity to treat waste.
Spare capacity being sold for commercial feed stocks could
seriously distort local market conditions, with below market prices
being used to secure feed stocks. An option might be that the
construction and/or operation of digesters at wastewater treatment
works is opened up to the commercial market; waste companies may or
may not be in a better position to deliver value for money through
this approach, which could result in lower operating cost and
perhaps ultimately a lower charge to the water company
customer.

Arriving at an appropriate financial framework will require
collaboration between OFWAT and other industry regulators and OFWAT
has sought guidance from the Office of Fair Trading for its view on
how to fairly regulate this issue. Water companies will require a
good financial model in order to determine a gate fee for waste,
which recognises: the income from the energy (and possibly
digestate disposal to land); the additional costs for handling the
waste and a contribution to the asset value that has been paid for
by the customer and thus should be recompensed). OFWAT have
made it clear that if the financial model is robust, they will not
stand in the way of co-digestion.

An important attraction of utilising existing water company
capacity is that they also have the assets and supply chains to
recycle the digestate. Without economic digestate recycling routes,
new digesters will struggle. Digestate is produced at 4% dry
solids and is thus expensive to transport. It must therefore
be dewatered to at least 20% dry solids. Dewatering liquor
must then be treated as it contains soluble N and P and some
organic matter. Water Companies have dewatering facilities
and a sewage treatment plant to put the liquor back through
(nutrient recovery as struvite and ammonia is emerging technology,
which is an added environmental advantage).

Prices for both energy and fertiliser are likely to increase in
the medium to long-term. Both biogas and fertiliser outputs
from AD represent more sustainable sources of these valuable
resources than fossil fuels and inorganic fertilisers, the prices
for which will become increasingly expensive by comparison.

The allocation of Renewables Obligation Certificates (ROCs)
vary considerably for sewage and biowaste (energy generated from
digested sewage sludge qualifies for 0.5 ROCs per MWh of
electricity generated) whereas that generated from biowaste
qualifies for 2 ROCs per MWh, making the allocation of ROCs for
mixed feedstock problematic. The Renewable Heat Incentive
provides 6.5p/kWh, guaranteed until 2035 for eligible schemes where
electricity is generated using CHP units.

Demographic issues

The benefits of existing AD capacity near to urban centres and
potential recycling routes for digestate are significant if the
regulatory hurdles can be overcome. It should be possible to
remove these hurdles, and importantly with no diminution in
environmental protection. Co-digestion of waste with sewage sludge
at existing wastewater treatment works (which are located close to
the major conurbations they serve and hence a ready supply of
biodegradable waste feedstock) offers an opportunity to achieve
financial viability for AD whilst reducing waste transportation
requirements (lowering the carbon intensity of the
collection/transportation process). For example, for sewage sludge
AD to be financially viable requires about 100,000 population, as
does food waste AD. Co-digestion would be viable for a town of
50,000 people but mono-digestion would not and wastes would have to
travel to a centralised site with the associated transport carbon
emissions. The water industry has a significant experience base on
AD, and to a lesser extent in relation to co-digestion.

Feedstock issues

There are a number of waste streams that can be anaerobically
digested, particularly domestic and commercial food wastes, and
commercial and industrial tankered wastes. The main issues
concern the quality of the feedstock and the ability to separate
contaminants, which can negatively affect digester efficiency, even
if present in relatively small amounts. Quality control of
feedstock is thus important, and becomes particularly relevant in
the case of source-segregated municipal solid waste where
contaminants are commonly found. The revised Waste
Framework Directive (WFD) may deliver improvements in the
segregation of waste which may improve feedstock quality
issues. A sister Biowaste Directive could also be
forthcoming, which may help to ease the regulation of
co-digestion.

The Animal By Products Regulations (ABPR)[xv] also have important
implications for co-digestion because of the proportion of other
materials introduced into sewage works AD plant, which may contain
meat or other waste animal material. Generally the presence
of such material in the feedstock requires heat treatment prior to
the digestion process. Defra has been working with Water UK to
provide guidance to water industry operators on the requirements of
these Regulations including at what point and in what proportions
pasteurized ABPR waste can be blended with sewage sludge. Advanced
digestion would meet ABPR requirements and would have the benefit
of trebling the capacity of many existing digesters. If this
treatment were applied to all co-digestion feed then the output
would be classified as enhanced treated digestate under the Safe
Sludge Matrix[xvi], but depending on the
detail of any agreement, this may not be required.

June 2011

Note: CIWEM Policy Position Statements (PPS) represent the
Institution's views on issues at a particular point in time. It is
accepted that situations change as research provides new evidence.
It should be understood, therefore, that CIWEM PPS's are under
constant review, and that previously-held views may alter and lead
to revised PPS's.